For about ten years, Big Oil moved away from Canada’s oil sands and into cheaper, more easily developed—and less regulated—locations. Now, the supermajors are returning, looking for a piece of an energy industry that just got a lot more attractive. Last week, Shell said it would buy Canada’s ARC Resources in a $16.4-billion deal that will add roughly 370,000 barrels of oil equivalent per day to its production and strengthen the supermajor’s position in one of the continent’s most strategic gas corridors. The…